A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
European Patent Application No 1093022 and corresponding U.S. Pat. No. 6,392,738 address cleaning of the substrate support table. The substrate support tables used in lithography have a support surface with protrusions that extend substantially perpendicularly from the surface. In operation, the backside of the substrate is supported on the protrusions, at a small distance from the support surface of the substrate table, in a position substantially perpendicular to the direction of propagation of the projection beam. Thus, the tops of the protrusions, rather than the support surface of the wafer table, define an effective support surface for the substrate.
EP1093022 and U.S. Pat. No. 6,392,738 describe that contamination present between the backside of the substrate and the protrusions can produce a deformation of the surface of the substrate with detrimental imaging effects. To solve this problem, these documents disclose a detector for recognizing the presence of contamination. Preferably, the presence of contamination is recognized from unevenness of the surface of a substrate on the substrate support table. A suitable device for sensing the level of the substrate is already available, since it is used to adjust the height and tilt of the substrate during exposure.
The documents also disclose cleaning tools for removing contamination from the protrusions. These cleaning tools include a ceramic cleaning block that can be positioned against the support surface and moved in parallel to it to remove contamination by abrasive cleaning. Another embodiment of the cleaning tool includes a sponge provided with a solvent for removing contamination. A further embodiment includes a laser for removing the contamination by heat treatment.
EP1093022 and U.S. Pat. No. 6,392,738 emphasize that measures should be taken not to damage the protrusions themselves when the contamination is removed. In the embodiment of the cleaning block, for example, the forces exerted by the cleaning block should not be so excessive as to damage the protrusions.
Unevenness of the supporting surface can also be caused by dissimilarity between the height of material that makes up the protrusion itself. This is typically the case when a new substrate support table has been manufactured. Possibly uneven wear may also lead to unevenness. The substrate support table typically contains a chuck on which the table with the protrusions is supported. Unevenness may be the result of differences between the height of the protrusions, or in the backside of the table or in the chuck. Therefore, these elements are carefully made level. Nevertheless, it has been found that unevenness may also result when the chuck and the support table (and any other elements) are assembled or installed.
A further problem with conventional support tables that include protrusions is that the wafer may stick to the protrusions. This compromises the reliability of the wafer handling.
Similar problems may be encountered with support tables for other articles that have to be supported in a well defined plane across the beam path, such as reflective masks or transmission masks.